1. Field of the Invention
This invention relates generally to apparatus for detecting and measuring the concentration of an energy absorbing compound in a breath sample of a human subject. More particularly, it relates to a method and apparatus for determining only ethanol concentrations in a breath sample without the influence of commonly occurring interferants, such as acetone and water vapor.
2. Description of the Prior Art
The basic physical principles of absorption by ethanol and other materials of energy from a beam of infrared electromagnetic energy, upon which the present invention is based, is well-known in the prior art. Such a technique has been fully described and illustrated in U.S. Pat. No. 3,562,524 which issued on Feb. 9, 1971 to Donald F. Moore and is incorporated herein by reference. Heretofore, there have been many methods and apparatus in the prior art which have utilized the principles of absorption by ethanol and made practical implementations of such infrared means to measure the ethanol concentration in a breath sample.
For example, in U.S. Pat. No. 3,792,272 to Harte et al there is disclosed a system for detecting and quantifying ethanol content in a breath sample which uses a single infrared wavelength (3.39 microns). Since this single wavelength of energy is absorbed both by ethanol and other energy absorbing compounds naturally occurring in the breath sample such as acetone or ingested compounds such as turpentine, the infrared measurement will be rendered inaccurate and overstated if other energy absorbing compounds are present.
Further, there is disclosed in U.S. Pat. No. 4,268,751 to Fritzlen et al and assigned to the same assignee of the present invention a method and apparatus for detecting the possible presence of an energy absorbing compound (i.e., acetone) in a breath sample which may render inaccurate a measurement of the concentration of a predetermined energy absorbing compound (i.e., ethanol) present in the sample. Fritzlen applies two predetermined wavelengths (3.39 microns and 3.48 microns) to the same breath sample contained in a chamber, at least one of which wavelength is sufficently absorbed by ethanol. The infrared energy remaining in each of the two wavelengths after absorption by the collected sample is received by an infrared detector which converts this remaining quantity of infrared energy to an equivalent electrical signal. The equivalent electrical signal representative of the first wavelength and the equivalent electrical signal representative of the second wavelength are continuously compared and their difference is required to remain substantially constant at a predetermined value throughout the test. The lack of a predetermined comparison value indicates the presence of an infrared energy absorbing compound other than ethanol. However, Fritzlen suffers from a disadvantage in that it merely detects the presence of an unknown energy absorbing compound but cannot determine accurately the concentration of the predetermined energy absorbing compound when both the unknown energy absorbing compound and the predetermined energy absorbing compound are present in a sample. Further, Fritzlen does not compensate for the presence of water vapor which is always present in a breath sample.
Accordingly, it would be desirable to provide a breath analyzer for determining the amount of a predetermined energy absorbing compound such as ethanol in a sample even when unknown energy absorbing compounds such as acetone and water vapor are also present. The present invention provides a method and apparatus for determining only ethanol concentrations in a breath sample without the influence of occurring interferants, such as acetone and water vapor. The technique of the present invention is so general in that the concentration of any desired energy absorbing compound found in a breath sample can be determined independently of any potential interferant.